The present invention relates to an apparatus and method for monitoring structural changes of an electrode in a rechargeable electrochemical cell battery, and, more particularly, to an in situ x-ray study apparatus and method utilizing an electrochemical cell holder having a structure and orientation which eliminates corrosion of the beryllium window of the x-ray study apparatus. Even more particularly, the invention relates to a rechargeable electrochemical cell battery structure for use in an in situ x-ray study apparatus.
The battery-monitoring apparatus of the present invention generally includes an electrolytic, or electrochemical cell comprising polymeric film composition electrodes and separator membranes. In particular, the apparatus includes a rechargeable electrochemical battery cell comprising an intermediate electrolyte/separator element containing an electrolyte solution through which ions from a source electrode material move between cell electrodes during the charge/discharge cycles of the cell. The invention is particularly useful for studying the operation of such cells in which one or both of the electrodes comprises a material capable of intercalating mobile ions, e.g., of sodium, potassium, and particularly lithium, and the electrolyte/separator membrane comprises a polymeric matrix made ionically conductive by the incorporation of an organic solution of a dissociable salt which provides ion mobility. Electrolytic battery cells of this type are generally described in U.S. Pat. Nos. 5,192,629 and 5,296,318, the disclosures of which are incorporated herein by reference, and in U.S. patent application Ser. No. 08/160,018, filed 30 Nov. 1993 now U.S. Pat. No. 5,460,904, issued 24 Oct. 1995, each of which is assigned to the assignee of this application.
The rechargeable battery industry is rapidly promoting the use of rechargeable Li-ion technology. In the Li-ion cell, the positive and negative electrodes comprise lithium intercalation compounds which can be viewed as host structures able to reversibly accept guest lithium ions. The structure of the host compound normally includes vacant sites which determine the number of lithium ions that the material can reversibly accept. The more such vacant sites, the greater the capacity (ampere/gram) of a battery cell using such an intercalation electrode. Thus, it is important for researchers working in the rechargeable battery field to have means for determining the factors limiting the number of lithium ions in an intercalation compound, for instance, the structural changes in the host material induced by intercalation or deintercalation of lithium during charge/discharge cycling.
With a technique that provides this information, one may monitor structural changes of intercalation materials as a function of voltage or electrode composition. Additionally, one may directly monitor the self-discharge mechanism within a battery cell, e.g., a LiMn.sub.2 O.sub.4 /electrolyte/C Li-ion cell, caused by the intercalation of lithium ions into the pure .lambda.-MnO.sub.2 phase. Such a technique may also be used to adjust the ratio of positive to negative electrode materials to ensure the optimum composition and efficient operation of a Li-ion system, since continuously monitoring the lattice cell parameters of the positive and negative electrode permits one to know at any time and at any voltage the population of lithium ions in each electrode. As a result, one can adjust the weight ratio of the electrodes in order for the positive electrode to be completely delithiated when the negative is fully lithiated.
An in situ x-ray diffraction apparatus and technique for performing such studies have been described, for example, by Dahn et al., "in situ X-ray diffraction experiments on lithium intercalation compounds", Can. J. Phys., 60 (1982), 307-313. A variation of in situ electrochemical x-ray study apparatus has also been described by Tarascon et al., "Electrochemical, Structural, and Physical Properties of the Sodium Chevrel Phases Na.sub.x Mo.sub.6 X.sub.8-y I.sub.y (X=S, Se and y =0 to 2)", J. Solid Sate Chem., 66 (1987), 204-224. In these prior procedures, where it is desired to study the effect of ion intercalation into an electrode material, the electrode layer is preferably disposed as directly as possible in the path of incident x-radiation in order to obtain strong data responses and avoid diffraction data contamination by extraneous cell components. This end was readily accomplished in these earlier systems by placing the electrode layer upon or directly in contact with a beryllium film serving the dual role of x-ray diffraction apparatus window and electrochemical cell current collector by means of which continuous cell cycling could be studied.
As the search for higher capacity and optimization of electrode materials continues, intercalation compounds with high-voltage regions above about 4.3 volts are becoming more important, since, for many layered intercalation materials, up to forty-five percent of the theoretical capacity lies in the region above this 4.3 volt threshold. Unfortunately, however, the study and analysis of these materials necessary for improving capacity have not previously been available with an in situ electrochemical cell x-ray study apparatus. A major problem, as described by Li et al., "In situ X-ray diffraction and electrochemical studies of Li.sub.1-x NiO.sub.2 ", Solid State Ionics, 67 (1993), 123-130, arises from the fact that, particularly when a positive cell electrode layer under study is placed in contact with the diffraction apparatus window, the beryllium film suffers extreme corrosion at cycling potentials in excess of about 4.3 volts, thus severely limiting the utility of the apparatus.
Due to this limitation, the structural variations in these potentially useful intercalation materials could only be determined in isolation by delithiating a different sample at each voltage level in an operating cell cycle. Useful test results could only be obtained with great difficulty in this manner and could seldom be correlated with any degree of reliability. The continuous, high-voltage method and apparatus of the present invention, however, have obviated these limitations and have enabled effective study and development of high capacity electrochemical cell electrode materials.